The background section for U.S. nonprovisional patent application Ser. No. 11/826,100 (“the '100 application”), titled “Circularly Polarized Low Wind Load Omnidirectional Antenna Apparatus and Method,” filed Jul. 12, 2007, which is hereby incorporated by reference in its entirety, addresses the opportunity introduced by opening previous television channels in the band from about 700 MHz to 750 MHz for new uses. These uses potentially include at least moderate-power broadcasting directed toward handheld receivers and related technologies and services. Broadcast antennas such as those presented in the '100 application address the requirements for wide-range coverage within this band, with a power level of 50 kilowatts effective radiated power (ERP) or less, in accordance with FCC regulations, as directed by 47 C.F.R. §27.50(c) and §27.55(a)(2).
Broadcast antennas such as those described in the '100 application are useful for economical provision of general coverage above local terrain. The relatively short wavelength dictates small size of each radiative element, associated with small clearance between conductive elements of differing potential. High power translates to high voltage, leading to risk of arcing and dielectric breakdown in such small elements. Large (high gain, multi-element) antennas permit reduction in the total number of transmitter sites needed to serve a region, but can introduce coverage (fill) and certain safety issues. Small (low gain, few element) antennas may require large numbers of transmitter sites to achieve coverage, aggravating logistical and cost issues.
Even assuming optimized antenna properties, however, use of the 700-750 MHz band introduces an additional consideration, namely building penetration. While very low frequency electromagnetic signals easily penetrate structures (and even sea water to a substantial depth), higher frequencies may be blocked. It is well known that the amplitude-modulated (AM) radio broadcast band, around 1 MHz, can be effectively blocked by ordinary structures such as overpasses, while the frequency-modulated (FM) radio broadcast band, around 100 MHz, has a small enough wavelength to permit usable signals to pass under the same overpasses, but can be impeded or blocked by tunnels and truss-style bridges, for example. All of these signals, as well as very-high-frequency (VHF) television channels, around 50-200 MHz, and lower ultra-high-frequency (UHF) television channels, around 470-700 MHz, pass readily through small amounts of wood, plaster, shingles, and like construction materials to provide signals within ordinary residential buildings, but tend to be stopped by structures having higher conductive content, such as steel-walled or -roofed business construction, steel-reinforced concrete floors, and the like.
The band from 700-750 MHz (former upper-UHF television channels 52-59) is likewise susceptible to such blockage, which can be of particular concern for sales outlets within shopping malls, for example. Ordinary commercial buildings frequently have steel construction, including trusses supporting metal roofs, steel walls, and one or more steel-reinforced concrete floors. Such arrangements provide significant, albeit incidental, shielding against broadcast signals, even when irregularly interrupted with windows and other unpredictable passages that may admit some signals.
Known types of broadcasting facilities are significantly deficient in broadcasting signals within propagation-impeding structures. For example, dipole radiators are sensitive to receiving antenna orientation. Patch antennas are highly directional, requiring attention to placement and orientation. Many antenna types are suitable for high power, and are priced accordingly. Overall systems solutions are significantly lacking: even though a desired signal may exist outside a building in substantially the desired form, such as circular polarization with a good power level, access within the building can be blocked.
Current systems are not capable of reliable delivery of one or more channels of broadcast signals inside a structure, where the structure exhibits an uncontrolled extent of shielding against signals present outside the structure. It is potentially useful to provide small, low-power broadcasting facilities within such propagation-impeding structures as adjuncts to ordinary broadcasting facilities in the vicinity.